The present invention relates to a coil system for magnetic resonance (MR) imaging, which is incorporated in a magnetic resonance imaging system for imaging information based on a density distribution of specific nuclear spins and/or a relaxation time of resonance at a specific portion of an object to be examined, and which is used to transmit an excitation pulse for exciting magnetic resonance and to receive a magnetic resonance signal generated by the magnetic resonance.
A uniform static magnetic field is applied to a predetermined portion to be imaged, i.e., a portion including a specific slice to be imaged of an object to be examined, a predetermined gradient magnetic field is applied to superpose the static magnetic field, and a predetermined selective excitation pulse, i.e., an RF (radio frequency) magnetic field (electromagnetic wave signal) for excitation is applied in a direction perpendicular to a direction of the static magnetic field, thereby generating an MR phenomenon only at the slice portion. In addition, after the excitation pulse is released, a magnetic resonance signal generated from the nuclear spin is received and Fourier-transformed, so that a spectrum of an angular frequency of a specific nuclear spin is obtained. A tomographic image based on the MR information, i.e., an MR image, can be obtained by image reconstruction based on a computed tomography (CT) technique. For example, according to a back projection technique, a slice to be imaged is excited to generate the MR phenomenon, and then a linear gradient magnetic field having an inclination with respect to an X' direction (a coordinate system rotated from an X-axis through .theta..degree.) is superposed on the static magnetic field, thereby obtaining an FID (free induction decay) signal. The FID signal is Fourier-transformed to obtain projection information. When projection information in each direction in an X-Y plane is obtained by rotating the X'-axis at the slice portion on the X-Y plane, the MR image can be reconstructed on the basis of this information.
In the MR imaging system for visualizing the MR image of an object to be examined in accordance with the above principle, a coil system comprising an RF coil is provided to transmit the excitation pulse as the RF electromagnetic wave signal to the object to excite the MR of nuclear spin or to receive the MR signal as the RF electromagnetic wave signal emitted from the nuclear spin by the MR. The RF coil is provided such that an RF magnetic field to be transmitted or received is positioned perpendicular to a main magnetic field. In a conventional system, the RF coil is constituted by a pair of saddle coils opposing each other across an imaging area where the object is placed.
An arrangement of a coil system in the conventional MR imaging system will be described below with reference to FIG. 1. Main magnetic field coil 1 serves as a magnet and generates static magnetic field B0 as a main magnetic field in a Z direction of FIG. 1 (normally in a direction coincident with a body axis direction of object P). Gradient magnetic field coil 2 generates slice-selecting (normally in the Z direction) and phaseencoding and/or reading (normally in an X and/or Y direction) gradient magnetic fields. RF coil 3 consists of a pair of saddle coils and is used to both transmit and receive the RF signal.
However, in the conventional system shown in FIG. 1, since coil 3 is constituted by the pair of coils opposing each other in a specific direction, only a component in a specific direction of the resonance magnetic field is received, thereby decreasing the reception efficiency of the MR signal. When the reception efficiency is low, an S/N ratio cannot be increased, and an image quality of the MR image is adversely affected and degraded by noise.